The present invention relates to method and apparatus for monitoring the quality of a tool on machine tool. Obtaining an optimum tool life e.g., a maximum volume of metal removed by the tool before resharpening, has been a problem of great concern in the industry for many years. Since excessive tool wear lead to high forces on the tool and this causes tool failure and interruption of production, it has been the practice to change the tools before this happens, thus, long before their useful life has been reached.
In order to cope with this problem, machines have been developed in which machining power is monitored while the feed rate under the tool is being controlled, so as to get fuller use out of the tools before they are resharpened. Thus, power as applied to the machine tool appears to be indicative of the cutting force exerted by the tool in operation. Power adaptive control may be used concurrently. After the machine spindle power has been initially established at a preset level, the table feed is automatically increased, or decreased, depending upon whether the machine power is below, or above, the preset power value. In one embodiment, an analog circuit establishes what the spindle power at a given instant actually is. The machine operator increases a potentiometer setting during cutting until, from direct observation, the tool appears to be becoming overloaded at which time the power level being reached is taken as the preset level. Thereafter, whenever the spindle power exceeds the preset level, that is when the tool encounters excessive uncut material, the table feed will be slowed down in order to restore the preset level. Conversely, should the tool start cutting lightly due to less material being in its path, then control will increase the table speed, thus maintaining the same preset level. See, for instance, U.S. Pat. No. 3,571,834 of Richard A. Mathias.
While this mode of power adaptive control works reasonably well for repeated rough cuts of the same geometry, with light finish cuts, characteristically, the control becomes less effective. In such case, the operator's setting for total power is only slightly greater than idle power, for instance 2.1 HP as compared to 2.0 HP. The operator could detect, by direct observation of the cutting, that a poor chip is being cut for the given spindle rpm. He would then decrease or increase spindle speed by the slight amount of control necessary to improve the chip quality. Unfortunately, when controlling the tool operation in this fashion, any change of spindle speed will also cause a change in idle power. Since the total power remains the same, the tool will in fact now cut heavier or lighter chips. The consequence will be that the tool can break, in the first alternative, or it will not cut at all, in the second alternative.
The improvement according to the present invention resides in providing means for automatically compensating for changes in the power required to idle the spindle. This makes the adaptive control easier to program and easier to use. The machine operator will no longer need to adjust the setting of the potentiometer as frequently. A direct consequence of such automatic compensation is to enable a numerical control (NC) programmer or a manufacturing engineer (ME) to calculate automatically the desired cutting power setting and readily set the potentiometer or digital setting accordingly, thus, without having to compensate for the resulting changes in idle power. This improvement is achieved by analog or digital means. When idle power compensation is achieved by analog means, a bias signal is supplied so as to nullify the output of the power transducer of the spindle motor when the spindle is turning at idle speed. Once the spindle is turning under load, the bias no longer matches the transducer output, and net power is derived which is used to monitor the tool sharpness or which can be applied within an adaptive control process. Where digital means obtain, a latch or memory circuit is used to store the output of the power transducer of the spindle motor while under idle conditions. Once the tool is in operation, the excess of power is obtained by subtracting the stored value from the transducer output, thereby to derive net power.
It is known from U.S. Pat. No. 3,220,315 of R. A. Mathias to derive an indication of the spindle torque after signal compensation in order to balance out spindle idle conditions. It is also known from U.S. Pat. No. 3,681,978 of R. A. Mathias, et al. to measure the external load applied to the tool by subtraction of idle conditions.
These techniques, however, are complicated and costly. Moreover, they are not conducive to full digital treatment as desirable with modern computerized numerical control. The preferred embodiment of the present invention approaches the problem of attaining a true and usable representation of the force exerted by the tool on the workpiece with its cutting edge at any torque-level or spindle speed, by using different parameters. Instead of using actual forces, as translated for instance into a mechanical deflection of the tool, or of inferring the torque from a consideration of spindle speed, it is proposed to use the electrical parameters of the power supply, namely voltage, current and phase angle (the latter where alternating current is involved) so as to directly derive an indication of the energy consumed both under cutting operation and under idle conditions. Net power, obtained by subtraction of an offset representing idle power, is taken as an indication of the amount of force developed at the interface of the tool with the workpiece. It is true that total power has been recognized in the prior art as indicative of the work done by the tool. However, total power has not been understood in the past as anything other than a related factor, and a practical determination of the force exerted by the tool has always called for sensors or other electromechanical devices for correlating parameters such as torque, speed, temperature, deflection, . . . .
Thus, power monitoring is effective to indicate the degree of wear in many situations, especially when the tool is used under closely controlled and identical cutting conditions. More sophistication is involved when the tool is used at several different occasions and under different successive machining operations. Still, it appears desirable and even necessary, when a major requirement is to ensure with a machine tool and optimum tool life, to determine wear of a tool as it occurs and in each situation to be able to ascertain whether tool failure is impending.
In accordance with the present invention, it is proposed to integrate the power consumed by a tool e.g., to derive an indication of the energy used by the tool over its lifetime. This concept is based on a formula, known in abrasive wear theory, relating the wear volume V to the load P and the sliding distance L of the tool to the workpiece, as follows: EQU V=K.multidot.P.multidot.L (1)
where K is a constant determined by chemistry, hardness, and other factors involving the tool and the workpiece. While P times L is equal to the work done, or the energy put into the wear process, it is conceivable that the wear volume should be proportional to the integrated power used in the wear process. Thus: EQU V=K.sub.2 .intg.(net power) dt (2)
Since the wear process may be somewhat sensitive to temperature and other parameters, equation (2) is an approximation. Nevertheless, it is found to be a good approximation. This formula also provides a good basis for further refinements. While constant K in formula (1) depends upon several factors affecting the tool and the workpiece, formula (2) is not so dependent and therefore is applicable to several different cutting situations. Moreover, equation (2) follows very advantageously from the aforementioned net power monitoring concept. Since net power monitoring rests upon the elimination of spindle idle power as a factor, net power which is the key parameter indicative of wear, can be integrated up to any given time and if such instantaneous integrations are effected at different successive time intervals and under successive different cutting situations, it becomes possible to keep track and continuously monitor wear, thereby to derive an overall tool wear indication. Equation (2) is also applicable to implementations other than through a direct derivation of net power electrically, where an indication of power is to be integrated with time. Thus, other known techniques for the determination of energy expanded through the tool can be used as a basis for integration under equation (2).
In accordance with the present invention, monitoring of the tool life is accomplished by combining in a machine tool (1) net power monitoring and (2) computerized machine control system. The net power monitor unit supplies a parameter to be processed for each tool while the computer unit associated with the machine tool keeps track of the accumulated usage for each of a plurality of tools and stands ready to give a warning message whenever a particular tool has to be replaced.
The net power circuit provides an instantaneous analog signal output proportional to the total power. A voltage-to-frequency converter generates a pulse train, the frequency of which is proportional to the total spindle power. Up/down counter means and offset storage means concur in providing a pulse signal having a frequency which is proportional to net power; e.g., cutting power. The pulse signal is accumulated in a counter. The computer unit automatically resets the counter to zero if another tool has to be inserted in the machine spindle, while the accumulated count is being held in reserve for later use as an initial count if the original tool is being put again into use on the machine. When resetting the counter, the offset storage is also reset if spindle speed and the tool have both been changed. Thus, the computerized control unit monitors the total accumulated pulse count for all tools used. A warning light, or buzzer, will be actuated whenever a tool has reached its total allowable count; e.g., an empirically predetermined optimum tool life. Should a tool be changed before it reaches such allowed total usage, the computerized control unit will do the bookkeeping, retaining count data and adding more in each instance to the past accumulated total for future use of the tool.
Moreover, a maximum allowable cutting power, or accumulated count, is assigned to each tool, which will trigger the warning and can bring about an immediate tool replacement. When such maximum is conservatively chosen, any unforeseen "catastrophic" situation due to excessive wear, can be avoided.
The invention also provides for additional refinements applicable in 20% of the situations. At high cutting power; e.g., when there is an increased rate of wear due to higher temperature, the cutting time can be adjusted by a weighting coefficient determined after test results. Since the cutting temperature increases with the square of the cutting speed at the spindle, the pulse train indicative of net power is squared and the result accumulated, thereby providing a total count which determines bookkeeping and decision-making.
By combining, according to the invention, net power monitoring and computerized control, great flexibility in the system design is achieved, thereby increasing the utilization factor of cutting tools. In the past, the variety of cutting situations encountered in the shop had made it difficult to use tool monitors and machine control because of the constantly changing tool usage conditions. The present invention permits closer control of tooling, and an improved determination of the useful life of the tools before resharpening.